A Surface Complexation Model for Adsorption of Rare Earth Elements onto Birnessite: Implication for the Adsorption of Hydrolysis Products of Rare Earth Elements in Alkaline Conditions

Birnessite is one of the dominant sorbents of rare earth elements (REEs) in aquifer sediments. A surface complexation model (SCM) for REE adsorption onto birnessite is proposed based on a previously published generic SCM of birnessite, with the addition of surface complexation reactions for hydrolysis products of REEs. The “nlls” optimizer based on the Powell’s nonlinear least squares method provided in PhreePlot was used to fit the SCM to the published experimental data. An iterated fitting procedure was conducted to estimate 56 binding constants for 14 REEs. At pH > 5, the fitted SCM reproduced the experimental data fairly well in calculating the percentages of adsorbed REEs as a function of pH. The modeling results showed that adsorption of REEs onto birnessite were mostly contributed by the formation of the hydrolyzed surface species in alkaline conditions (pH > =8).